The migration of blood clot from the peripheral vasculature to the pulmonary arteries and lungs is known as pulmonary embolism. Typically, these clots originate in the lower limbs and migrate toward the heart and lungs. These clots can result from a variety of conditions such as trauma or deep vein thrombosis. If a clot is of sufficient size, it can occlude the pulmonary arteries and interfere with blood oxygenation in the lungs. This occlusion can result in shock or death. Individuals who experience a pulmonary embolism have a high likelihood of experiencing subsequent embolic events.
In these cases, blood thinning medications, e.g., anticoagulants such as heparin and warfarin sodium, or antiplatelet drugs such as aspirin, are given to the patient to prevent another embolic event. The utility of these medical therapies is limited because they may not be able to be administered to patients following surgery or stroke or for those patients presenting with a high risk of internal bleeding. Additionally, these medications are not always effective at preventing recurrent embolic events.
Therefore, surgical methods were developed in an effort to reduce the likelihood of pulmonary embolism recurrence by physically blocking the blood clot from migrating to the pulmonary artery and lungs. Since the inferior vena cava transports blood from the lower limbs to the heart, this vessel was a common site of surgical intervention. One method of treatment involved reducing the size of the inferior vena cava by application of ligatures or clips around the vessel. This prevented the migration of large clots from the lower vasculature to the heart. However, this required an extensive open surgical procedure with associated abdominal incision and general anesthesia. The effects of the surgical procedure coupled with lengthy recovery times led to complications such as vessel thrombosis and lower extremity swelling; thereby aggravating the condition of the patient.
To avoid this invasive surgical approach, less invasive catheter-based approaches have been developed. These involve the placement of filter devices in the inferior vena cava. These filters are inserted under local anesthesia through the femoral vein in the patient's leg, the right jugular vein in the patient's neck or the subclavian vein in the patient's arm. Using standard catheter techniques, the filters are then advanced intravascularly to the inferior vena cava where they are deployed and expanded against the vessel wall. These filters interrupt the migration of blood clots from the lower extremities to the heart and lungs. Once trapped in the filter, flow of blood around the clot helps to dissolve the embolic load in the device.
Previous filters take various forms. One type of filter is comprised of coiled or looped wires such as disclosed in U.S. Pat. Nos. 5,893,869 and 6,059,825. Another type of filter consists of legs with free ends having anchors for embedding and stabilizing in the vessel wall. Examples of these filters are disclosed in U.S. Pat. Nos. 4,688,553; 4,781,173; 4,832,055; 5,059,205; 5,984,947 and 6,007,558. Finally, filters that incorporate a means for removal are disclosed in U.S. Pat. Nos. 5,893,869; 5,984,947 and 6,783,538. U.S. Pat. No. 6,635,070 describes a temporary filter device that is removed by everting a portion of the filter structure to allow it to be withdrawn into a catheter device.
Several factors need to be considered in designing filters for use in the venous system. To prevent migration to the heart, the filter must be securely anchored to the adjacent vessel wall. However, filter anchoring must be accomplished in an atraumatic fashion so as to avoid vessel wall damage and perforation of the neighboring descending aorta and bowel. The area of contact with the vessel wall should be minimized in order to avoid vessel wall hypertrophy and caval stenosis. In addition, the filter must be capable of collapsing to an acceptable delivery profile to allow atraumatic intravascular delivery to the inferior vena cava. Additionally, the filter should direct blood clots away from the vessel wall to avoid vena cava thrombosis. Finally, it is preferred that such a filter device be removable from the implant site.
Three key shortcomings of current vena cava filter designs include: (1) inability or difficulty of filter removal, (2) non-optimal flow characteristics resulting in flow stasis, flow stagnation and filter occlusion and (3) caval stenosis. From a clinical perspective, there are many instances in which it would be desirable to place a venous filter in a patient on a prophylactic basis and then remove the filter when it is no longer required, e.g. young trauma patients, obese patients, or neurosurgical patients. In addition, current venous filters do not exhibit an optimized flow pattern in the presence of clot. It would be advantageous to develop a filter that distributes captured clot in such a way as to minimize significant central (mid-line, or about the longitudinal axis of the vessel) flow disturbances and avoid clot contacting the vessel wall. Finally, the hypertrophic tissue response in the regions of the vessel wall contacted by the filter device not only inhibits filter removal but also causes stenosis of the vena cava. This vessel stenosis can lead to thrombosis of the vena cava.